In common rail fuel injection systems, fuel injectors (typically four, six or eight) are provided to inject fuel at high pressure into the associated combustion cylinders. Each fuel injector includes an injection nozzle having a valve needle which is operated by means of an actuator to move towards and away from a valve seating so as to control fuel delivery by the injector.
A known injection nozzle is shown in FIG. 1, and such an injection nozzle may be incorporated within a direct-acting piezoelectric injector such as described in EP0955901, and is also applicable to indirect-acting, or ‘servo’, injectors.
With reference to FIG. 1 an injection nozzle 2 includes a metal nozzle body 4 that defines a cylindrically-shaped blind bore 6 within which a needle-like valve member 8 is slidable. The blind end of the nozzle bore 6 is defined by a conical seating surface 10 which blends, in a radially inward direction, into a sac volume 12. The nozzle body 4 also includes a plurality of nozzle outlet passages 14 (two of which are shown), the inner ends of which open through the wall of the sac volume 12.
The valve needle 8 includes a generally cylindrical (upper) region 15 and a generally conical tip section 16 that is sealingly engageable with the seating surface 10. For this purpose, the tip section 16 defines a frustoconical upper region 18 having a cone angle less than that of the conical seating surface 10 and a lower conical region 20 that sits below the upper region 18 and has a cone angle greater than the conical seating surface 10.
The intersection between the upper and lower tip regions defines a seating line 22 that is engageable with the conical seating surface 10, the seating line 22 defining a precise annular engagement point which ensures an effective and durable seal is achieved.
Note that the cylindrical region 15 of the valve needle 8 is slimmer than the surrounding part of the bore 6 so as to define a chamber 24 for fuel, hereinafter referred to as the ‘delivery chamber’.
The valve needle 8 is moveable axially, along the longitudinal axis of the injection nozzle 2, so as to control fuel from the outlet passages 14. In use, as the valve needle 8 is moved upwardly, in the orientation shown in FIG. 1, the tip section 16 disengages the seating surface 10 so that high pressure fuel present in the delivery chamber 24 can flow past the tip section 16, into the sac volume 12 and through the outlet passages 14 into an associated combustion chamber. Re-engagement of the tip section 16 with the seating surface 10 closes the outlet passages 14 thus terminating fuel injection.
The Applicant has observed that in such prior art nozzles, during low lift conditions (up to approximately 35% of maximum lift) the fuel flowing past the seating region tends to follow or “stick” to the surface of the valve needle into the sac volume before turning tightly to enter the outlet entry openings. However, as the lift of the valve needle increases and the opening into the sac volume becomes larger, the flow switches to follow or ‘stick’ to the surface of the sac volume. However, the switching phase from the flow following the valve needle to following the sac surface is not instantaneous and this transition is characterised by a period of flow instability during which period the flow can flap or toggle between following the valve needle or sac wall surface and, in the worst case, this creates a chaotic flow regime in the sac volume.
The flow instability phenomenon described above can encourage nozzle outlet-to-outlet spray variation and also can cause some variation on the axially upward force acting on the valve needle which can lead to hesitation of the valve lift and further spray fluctuations. Furthermore, the shot-to-shot fuel delivery accuracy of the injector is compromised
In general, the above factors lead to higher levels of polluting emissions, and a reduction in engine performance and efficiency.